In order to study the crack resistance of prestressed composite beams, two groups of 8 test beams were designed and fabricated, and the influence of prestressing level and section aspect ratio on crack and crack characteristics was analyzed.The results show that the average cracking load of the two groups of composite beams increased by 42% and 23% respectively after prestressing, and the effect of delaying crack development was significant, but the effect weakened after the prestressing level was higher. It is important to increase the ratio of section width to section height to improve the cracking moment and crack resistance of prestressed composite beams, which can greatly increase the ultimate bearing capacity. Combined with the existing theory and test results, the calculation method of cracking load is proposed.